Unique DNA-synthesizing enzymes (TdT, ADA, PNP and MTAPase) in precursors of lymphoid cells serve as tumor markers. We have prepared monoclonal antibodies to TdT and ADA and have developed immunoperoxidase staining and enzyme-linked immunoassays. Both ADA and PNP have been studied as potential targets for chemotherapy in lymphoproliferative diseases since genetic deficiencies of these enzymes are associated with profound lymphopenia.